The government is paying for some of the most advanced research into controversial face-recognition technology, which converts photos into numerical sequences that can be instantly compared with millions of photos in a database.

Facial-recognition research was sought to enable federal air marshals to surreptitiously photograph people in airports and bus and train stations to check whether they were on terrorist databases. The air marshals disavowed the technology to focus on identifying suspects through methods that don’t use cameras.

Even so, the research continues and could help police identify someone photographed by a security camera, said David Boyd, head of command, control and interoperability at Homeland Security’s Science and Technology branch.

The technology has been tested at Boston’s Logan International Airport, in busy city centers and at the 2001 Super Bowl in Tampa

The ability to establish quick identities will “turbocharge video surveillance,” ACLU privacy expert Jay Stanley warns. “It turns ‘dumb’ camera lenses into ‘smart’ observers that not only capture images but attach an identity to the image. That could increase the attractiveness of surveillance cameras.”

Melissa Ngo of the Electronic Privacy Information Center says the technology endangers privacy by enabling ordinary security cameras to find out the names of people being observed. “Why are you being tracked if you’re not doing anything wrong?” she said.

Face-recognition cameras have helped casinos spot known card counters and other unwelcome gamblers, said Walter Hamilton, chairman of the International Biometric Industry Association.

More recently, 19 states have adopted the technology and compare driver’s-license applicants with a photo database of license holders to see whether an applicant already has a license or is using a false identity, Hamilton said.

The Homeland Security research aims to make the technology work in one area where it has failed: surveillance. Tampa and Virginia Beach police removed face-recognition systems that did not yield a single arrest. During a test at Boston’s airport in 2002, the system failed 39% of the time to identify volunteers posing as terrorists at security checkpoints.

Using face-recognition for surveillance is “enormously difficult” because systems photograph people at oblique angles or in weak light, both of which create poor images, said Takeo Kanade, head of Carnegie Mellon University’s Robotics Institute. Terrorists can defeat the systems with disguises or hats that shield their faces.

The Homeland Security research aims to counter shortcomings by creating technology that will “take a partial picture of a face and reconstruct that into a full frontal shot,” Boyd said. “No one has done that before.”

Kanade said the research, by L-1 Identity Solutions of Stamford, Conn., “challenges the most difficult part of face recognition. It’s a challenge worth pursuing.”